One of the problems encountered in interfacing a signal transmitter and a signal receiver with the same end of an optical fiber is the attenuation undergone by the transmitted and received beams in an optical coupler designed to separate these beams from each other. Such couplers conventionally include semireflective mirrors which act as beam splitters. A system of this type is described, for example, in German Published Specification (Offenlegungsschrift No. 2,546,269; the semireflector of that system is provided with a fully transparent central aperture in a common focus of three converging lenses two of them lying on the fiber axis while the third one lies on a line perpendicular thereto. It is also known to use Y-shaped fiber junctions for beam-splitting purposes.
In all these prior systems the attenuation of the luminous energy is relatively high. Since a collective lens generally introduces a loss of about 1 dB. the successive traverse of two cascaded lenses in a system such as that of the aforementioned German publication accounts for a power loss of about 2 dB which is particularly undesirable in the case of a received beam.
Another drawback of conventional two-way couplers is the limitation imposed by the acceptance angle of the fiber upon the amount of luminous energy that can be transmitted.